The present invention relates to an air foil nozzle dryer for drying webs of material, comprising two nozzle arrangements arranged opposite one another relative to the web of material passing between, said web of material being supported between these nozzle arrangements without contact.
In such air foil nozzle dryers the web of material is held in a hovering position and dried by the air streams emitted by the nozzles of the nozzle arrangements. The efficiency of an air foil nozzle dryer largely depends on the distance between the nozzles and the web of material. The closer the nozzles are located to the web of material, the more rapid is the breaking-up process of the boundary film required for the initiation of the drying process. However, it should be noted in determining the distance between the nozzles that, depending on the property of the web material and the distance between the latter and the nozzles, the hovering web of material may be undesirably aspirated to the nozzles (Bernoulli effect). This is indicated principally by a fluttering motion of the web of material, a phenomenon that occurs mainly along the edges.
In certain air foil nozzle dryers of the conventional type drawing-in apparatus are provided, which renders it necessary to make the distance between the nozzles large enough for the drawing-in apparatus to be moved between the nozzles. This in turn involves the disadvantage that the comparatively great distance between the nozzles and the web of material results in an extension of the drying time and/or in an increase in energy requirement for the reasons outlined above.
In addition, the drying process in the known air foil nozzle dryers can take place under optimal conditions only with webs of a material with specific properties. In the case of webs of material of which the properties of the material (areal weight, flexural stiffness etc.) deviate the said fluttering effect is observed to a greater or lesser degree. In order to avoid that, complex nozzle designs have so far been provided (see e.g. Swiss patent specification No. 542.080).